Field of the Invention
The invention relates to a current transformer which is used for current strength measurement on/in electrical devices that are at high/medium voltage, such as, for example, overhead power transmission lines, cables, substations, transformers and the like.
A current transformer with which the invention is concerned is constructed with surface-wave technology, that is to say it has as its essential component an electroacoustic surface-wave configuration with surface-wave structures.
Surface-wave configurations relevant to the invention are disclosed by Published International Patent Applications WO 93/13495 and WO/CH93/00252 and U.S. Pat. Nos. 3,273,146; 4,725,841 and 4,620,191. The cited applications and patents describe methods of building and operating various surface-wave configurations, all of which have the common factor that, through the use of an interdigital transducer, surface waves are produced in a piezoelectric substrate body from an electrical signal, and those surface waves as a rule propagate essentially perpendicularly to the interdigital alignment of the transducer electrodes at the surface of the substrate body. It is possible to recover a characteristically altered radiofrequency signal from the surface wave with a second interdigital transducer, which may even be the transducer already described above, in double-mode operation. As described in the prior art, such a surface-wave configuration may include even further structures such as reflector structures, further transducer structures, etc. which, for example, may also have a dispersive configuration of reflector/transducer fingers, an encoded configuration of fingers, and the like.
An essential aspect and subject of the present invention is that the current transformer has a radio-interrogated sensor part made by using surface-wave technology, which is passive, that is to say it requires no (DC) power supply, and to which a radiofrequency signal, for example a burst pulse, an FM-CW signal, a chirped pulse and the like, is transmitted from a remote radiofrequency transmitter. For that purpose, the surface-wave configuration of the sensor part, that is to say its input transducer structure, is equipped with an antenna for radio reception of that transmitted pulse. A corresponding antenna, which is connected to a second transducer structure of the surface-wave configuration, or, in the case already mentioned of a transducer structure with double-mode operation, is the same antenna, is used to transmit back the impulse-response signal of the surface-wave configuration, which is to be received in a remote receiver. The impulse-response signal that is transmitted back as a rule is different from the signal received by the surface-wave configuration, namely according to the current-strength measured value to be determined, and that is actually because of corresponding physical action on the surface-wave configuration.
Radio-interrogated surface-wave configurations are already used, for example in toll systems on roads, in road tunnels and the like, but in that case the detection of preprogrammed individual encoding of the impulse-response signal for object identification is involved. Radio-interrogated surface-wave configurations have also been used in metrology. Those configurations as a rule are constructed as delay lines, and measures are taken for the purpose of measurement in such a way that the measured quantity to be determined in the surface-wave configuration causes a change in the propagation time of the acoustic wave. That change in propagation time may be based on an electric field (oriented transversely to the propagation direction of the surface wave) in the substrate body, with that field producing, for example by piezoelectric effect, a change in propagation time in the corresponding partial region of the substrate body (Published European Patent Application 0 166 065 A1). By way of example, a temperature sensor using a change in the wave propagation time is known (Published European Patent Application 0 465 029 A1, corresponding to U.S. Pat. No. 5,286,109). A configuration which exploits an impedance change of an organic layer applied on the surface of the substrate body of the surface-wave configuration is suitable for measuring surface loading of that layer, for example with a chemical substance to be identified/quantitatively measured (Electronics Letters, Vol. 23 (1987) No. 9 pp. 446/447). A relevant pressure meter is also known, in which the mechanical property of the body, for example flexion, altered as a function of pressure in the material of the substrate body of the surface-wave configuration, causes a change in the propagation time of the acoustic wave and renders it usable for determining the measured value (Proceedings IEEE, Vol. 64, (1976) pp. 754-6). However, in the case of the configurations last mentioned herein, a remote interrogation by radio is not provided. Magnetosensitive sensors per se are likewise known in a different context, for example from German Published, Non-Prosecuted, Patent Application 38 28 005 and from a paper entitled "Magneto-Resistive Sensoren, Grundlagen . . ." [Magnetoresistive Sensors, Fundamentals . . ."] in Proceedings of the Symposium of Jun. 25, 1992, Dortmund, Dienstleistungszentrum MIOK, Institute for Applied Physics, University of Jena.